Image forming apparatus

ABSTRACT

An image forming apparatus includes a rotating polygon mirror configured to rotate and reflect the first laser beam and the second laser beam, a first generation unit configured to generate a first signal based on timings at which the respective first laser beams, a second generation unit configured to generate a second signal for determining a timing to form an electrostatic latent image on the second image bearing member with the second laser beam based on the first signal and a correction value, and a calculation unit configured to calculate the correction value based on the output of the detection unit, in which the calculation unit calculates the correction value based on the signal output from the detection unit in a period of time that has passed until the rotating polygon mirror reaches steady rotation since it has started to be driven to rotate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus.

2. Description of the Related Art

Conventionally, a color-image forming apparatus using anelectrophotographic system has performed image formation by reflecting alaser beam modulated in response to an image signal from a polygonmirror serving as a rotating polygon mirror and scanning aphotosensitive drum with the reflected laser beam to form anelectrostatic latent image. In such an image forming apparatus, when aplurality of scanners including light sources for laser beams in colorsis used, a main body of the apparatus increases in size and theapparatus increases in cost.

To miniaturize the apparatus and reduce the cost thereof, aconfiguration in which a common scanner is used for a plurality ofcolors has been known. Furthermore, Japanese Patent ApplicationLaid-Open No. 4-313776 discusses using a common scanner for a pluralityof colors and providing a beam detect (BD) sensor for one of a pluralityof light sources.

Japanese Patent Publication No. 4393133 discusses measuring, if an errorα occurs on each of mirror surfaces of a polygon mirror (see FIG. 8A), aperiod of a BD signal when the mirror surface of the polygon mirror isscanned with a laser, using a BD sensor, and calculating a correctionvalue for each mirror surface to correct the error.

By adding the correction value, it is possible to generate an accurateBD signal for light sources other than a light source provided with theBD sensor even if a plane division error occurs on each of the mirrorsurfaces of the polygon mirror. The BD signal for the light source otherthan the light source provided with the BD sensor is referred to as apseudo BD signal.

In a configuration in which the pseudo BD signal is generated, discussedin Japanese Patent Publication No. 4393133, a scanner is started after aprint instruction is received so that the polygon mirror converges at arotation speed for performing image formation (a steady rotation state),and a correction value for generating the pseudo BD signal is thencalculated. Then, the image formation is started. Thus, the start of theimage formation is delayed by a period of time during which thecorrection value for generating the pseudo BD signal is calculatedcompared with a configuration in which the pseudo BD signal need not becorrected. As a result, a first print output time serving as a period oftime that has passed until image formation on the first recording mediumis completed since the print instruction has been received is extended.

SUMMARY OF THE INVENTION

The present invention is directed to suppressing extension of a firstprint output time. The present invention is directed to providing animage forming apparatus including a first light source configured toemit a first laser beam and a second light source configured to emit asecond laser beam, a first image bearing member and a second imagebearing member configured to bear a developer image, a rotating polygonmirror configured to rotate and reflect the first laser beam and thesecond laser beam to scan the first bearing member with the first laserbeam and scan the second bearing member with the second laser beam, amirror surface reflecting the second laser beam is different from amirror surface reflecting the first laser beam at the same time, adirection of reflecting the second laser beam is different from adirection of reflecting the first laser beam at the same time, a firstgeneration unit configured to generate a first signal for determining atiming to form an electrostatic latent image on the first image bearingmember with the first laser beam based on timings at which therespective first laser beams, which have been reflected from the mirrorsurfaces, are sequentially detected by the detection unit, a secondgeneration unit configured to generate a second signal for determining atiming to form an electrostatic latent image on the second image bearingmember with the second laser beam reflected from the mirror surfacedifferent from the mirror surface reflecting the first laser beam basedon the first signal and a correction value, in which the calculationunit calculates the correction value based on the signal output from thedetection unit in a period of time that has passed until the rotatingpolygon mirror reaches steady rotation since it has started to be drivento rotate.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a schematicconfiguration of a printer according to the present exemplaryembodiment.

FIG. 2 is a schematic perspective view illustrating a configuration of ascanner unit in the present exemplary embodiment.

FIG. 3 is a block diagram for illustrating a configuration in a firstexemplary embodiment.

FIG. 4 is a timing chart for illustrating an operation in the firstexemplary embodiment.

FIG. 5 is a block diagram illustrating a circuit configuration of anapplication specific integrated circuit (ASIC) in the first exemplaryembodiment.

FIG. 6 is a timing chart for determining respective positions ofpolygonal surfaces by circuits in the ASIC.

FIGS. 7A and 7B are timing charts for illustrating a print sequence.

FIGS. 8A, 8B, and 8C illustrate the acceleration of a polygon mirror.

FIG. 9 is a graph illustrating a relationship between a speed and a timeat the time when the polygon mirror is driven to rotate.

FIG. 10 is a flow of pseudo BD control during the startup of a scanner.

FIG. 11 illustrates a configuration of a system for generating a pseudoBD signal in a second exemplary embodiment.

FIG. 12 is a block diagram illustrating a circuit configuration of anASIC in the second exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments for implementing the present invention will bespecifically described below with reference to the drawings. However,dimensions, materials, and shapes of components described in theexemplary embodiment and their relative arrangement are to be changed,as needed, according to a configuration of an apparatus for which thepresent invention is employed and various conditions. That is, the scopeof the present invention is not intended to be limited to theembodiments, described below.

<Image Forming Apparatus>

A color laser printer (hereinafter referred to as a printer) 201 servingas an image forming apparatus according to a first exemplary embodimentwill be described below with reference to FIG. 1. FIG. 1 is a schematicsectional view illustrating a schematic configuration of the printeraccording to the present exemplary embodiment. The printer 201 isconnected to a host computer 202 and a server (not illustrated). Theprinter 201 includes four image forming units to form a color imageobtained by overlapping images in four colors (yellow Y, magenta M, cyanC, and black BK).

The image forming unit includes toner cartridges 207 to 210 respectivelyhaving photosensitive drums 301 to 304 serving as image bearing members,and a scanner unit 205 having a laser diode for generating a laser beamas a light source for image exposure (a light source).

The printer 201 starts a printing operation when it receives a printinstruction and image data from the host computer 202. Specifically,first, each of operation portions in the printer 201 starts a startupoperation. Simultaneously, a video controller 203 in the printer 201rasterizes the received image data into desired video signal formationdata (e.g., bit map data), to generate a video signal for imageformation. When the startup of each of the operation portions in theprinter 201 is completed, the video signal is transmitted to an enginecontroller 204. The video controller 203 and the engine controller 204transmit and receive information via serial communication. The enginecontroller 204 drives the laser diode in the scanner unit 205 inresponse to the video signal. Thus, respective electrostatic latentimages are formed on the photosensitive drums 301 to 304 surfaces ofwhich have been charged by charging rollers (not illustrated) serving ascharging means in the toner cartridges 207 to 210.

The photosensitive drums 301, 302, 303, and 304 are used to formrespective electrostatic latent images in black BK, cyan C, magenta M,and yellow Y. In the respective toner cartridges 207 to 210, respectivetoners (developers) are supplied so that the electrostatic latent imagesformed on the photosensitive drums 301 to 304 are visualized(developed), to form toner images (developer images) on thephotosensitive drums 301 to 304. Among the toner images in black BK,cyan C, magenta M, and yellow Y formed on the photosensitive drums 301to 304 (image bearing members), the image in yellow Y is firsttransferred onto an intermediate transfer belt 211 serving as an endlessbelt capable of performing cyclic movement, and the images in magenta M,cyan C, and black BK are sequentially transferred in this order to besuperimposed on the image in yellow Y (primary transfer). Thus, a colorimage is formed on the intermediate transfer belt 211. The intermediatetransfer belt 211 contacts each of the photosensitive drums 301 to 304,to form a transfer nip.

In the toner cartridges 207 to 210, development devices 309 to 312serving as development means and cleaning devices 305 to 308 arerespectively disposed. A recording material such as paper in a cassette314 is fed to a registration roller 319 by a feeding roller 316, and isconveyed in synchronization with the color image on the intermediatetransfer belt 211 depending on a timing at which the registration roller319 is driven. A transfer roller 318 transfers the color image onto therecording material from the intermediate transfer belt 211 (secondarytransfer). The recording material on which the color image has beentransferred is conveyed to a fixing device 313. The fixing device 313fixes the color image onto the recording material with heat andpressure. Then, the recording material on which the color image has beenfixed is discharged onto a discharge tray 317 in an upper part of theprinter 201.

The printer 201 includes a registration detecting sensor 212 thatmonitors a registration position of the color image on the intermediatetransfer belt 211. The registration detecting sensor 212 reads aposition of the image in each of the colors formed on the intermediatetransfer belt 211 at a desired timing other than the time when imageformation is performed, and feeds back data representing the position ofthe image to the video controller 203 or the engine controller 204.Thus, color shift can be prevented by adjusting the registrationposition of the image in each of the colors.

<Scanner Unit>

Details of the scanner unit 205 in the present exemplary embodiment willbe described below with reference to FIG. 2. FIG. 2 is a schematicperspective view illustrating a configuration of the scanner unit 205 inthe present exemplary embodiment. Laser diodes LD1 to LD4 illustrated inFIG. 2 respectively scan the photosensitive drums 301 to 304 based on avideo signal generated by the video controller 203. The laser diode LD4corresponds to a first light source according to the present invention,and laser diodes LD1 and LD2 correspond to a second light sourceaccording to the present invention. The photosensitive drum 304corresponds to a first image bearing member according to the presentinvention, and the photosensitive drums 301 and 302 correspond to asecond image bearing member according to the present invention.

A polygon mirror (which may be hereinafter referred to as a polygon) 105serving as a rotating polygon mirror having a plurality of mirrorsurfaces rotates in a direction indicated by an arrow A in FIG. 2 usinga motor (not illustrated), and deflects and scans respective laser beamsemitted from the laser diodes LD1, LD2, LD3, and LD4. A motor, whichdrives the polygon mirror 105 to rotate, rotates by being controlled tofall within a predetermined speed range in which image formation can beperformed in response to an acceleration signal and a decelerationsignal in a speed control signal (not illustrated) from the enginecontroller 204 illustrated in FIG. 1.

The polygon mirror 105 reflects respective laser beams (first laserbeams) emitted from the laser diodes LD3 and LD4 on the mirror surfacewhile reflecting respective laser beams (second laser beams) emittedfrom the laser diodes LD1 and LD2 on the mirror surface different fromthe mirror surface from which the laser beams from the laser diodes LD3and LD4 are reflected. In the case, the polygon mirror 105 reflects thelaser beams emitted from the laser diodes LD1 and LD2 in a directiondifferent from a direction in which the laser beams emitted from thelaser diodes LD3 and LD4 are reflected. Specifically, the polygon mirror105 irradiates the laser beams emitted from the laser diodes LD3 and LD4onto the photosensitive drums 303 and 304 and scans the irradiated laserbeams while irradiating the laser beams emitted from the laser diodesLD1 and LD2 onto the photosensitive drums 301 and 302 and scans theirradiated laser beams. The laser beams emitted from the laser diodesLD1 and LD2 and the laser beams emitted from the laser diodes LD3 andLD4 enter the different mirror surfaces of the polygon mirror 105 at thesame timing.

A BD sensor 110 illustrated in FIG. 2 is arranged at a predeterminedposition which the laser beam from the laser diode LD4, which has beenreflected in a predetermined direction by the polygon mirror 105,enters. The BD sensor 110 receives (detects) the laser beam from thelaser diode LD4, and outputs a horizontal synchronizing signal (BDsignal) serving as a first signal based on the received laser beam. Thehorizontal synchronizing signal output by the BD sensor 110 is a signalfor determining a timing to form the electrostatic latent image on thephotosensitive drum 304, i.e., a signal for determining a timing to emitthe laser beam from the laser diode LD4. The BD sensor 110 correspondsto a detection unit and a first generation unit according to the presentinvention.

The laser beam emitted from the laser diode LD4 is scanned by therotation of the polygon mirror 105 while being reflected by the polygonmirror 105, and is further reflected from a folding mirror 109, to forman image on the photosensitive drum 304. Thus, the electrostatic latentimage is formed on the photosensitive drum 304. Actually, the laser beampasses through various types of lens groups (not illustrated) to befocused on the photosensitive drum 304 or converted into parallel lightfrom diffusion light.

Generally, the video controller 203 illustrated in FIG. 1 transmits avideo signal to the engine controller 204 after a predetermined periodof time has passed after detecting an output signal of the BD sensor110. Thus, the image by the laser beam on the photosensitive drum 304always starts to be written at the same position in a main scanningdirection. The main scanning direction means a longitudinal direction ofthe photosensitive drum.

On the other hand, the laser diodes LD1, LD2, and LD3 respectively formthe electrostatic latent images on the photosensitive drums 301, 302,and 303, similarly to the laser diode LD4. The BD sensor 110 is providedon only a scanning optical path of the laser beam from the laser diodeLD4, and does not exist on respective scanning optical paths of thelaser beams from the laser diodes LD1, LD2, and LD3.

The respective laser beams from the laser diodes LD3 and LD4 enter thesame mirror surface of the polygon mirror 105 at the same timing. Thus,a BD signal output from the BD sensor 110 can be used as a horizontalsynchronizing signal serving as a reference of a timing to emit thelaser light from the laser diode LD3 for forming the electrostaticlatent image on the photosensitive drum 303. On the other hand, therespective laser beams from the laser diodes LD1 and LD2 enter themirror surfaces different from the mirror surface of the polygon mirror105 which the light from the laser diode LD4 enters at the same timing.

In the present exemplary embodiment, an application specific integratedcircuit (ASIC) 402 (see FIG. 3) generates BD signals for the laserdiodes LD1 and LD2. A horizontal synchronizing signal serving as asecond signal generated by the ASIC 402 is a pseudo horizontalsynchronizing signal (hereinafter referred to as a pseudo BD signal) fordetermining timings to form the respective electrostatic latent imageson the photosensitive drums 301 and 302.

In the present exemplary embodiment, the laser beams from the laserdiodes LD1 and LD2 enter the same surface of the polygon mirror 105 atthe same timing. Thus, a common pseudo BD signal can be used for thelaser diodes LD1 and LD2. In the following description, the ASIC 402generates the pseudo BD signal for the laser diode LD2. The ASIC 402corresponds to a second generation unit according to the presentinvention.

As described above, a toner image in yellow Y by the laser diode LD4including the BD sensor 110 is formed on the photosensitive drum 304.Toner images in black BK, cyan C, and magenta M by the laser diodes LD1,LD2, and LD3 including no BD sensor 110 respectively are formed on thephotosensitive drums 301, 302, and 302. Thus, image formation isperformed. The foregoing is a series of processes for the imageformation.

<Method for Generating Pseudo BD Signal>

A method for generating a pseudo BD signal will be described below withreference to FIG. 3. FIG. 3 is a block diagram for illustrating aconfiguration in the first exemplary embodiment. The engine controller204 includes the ASIC 402 and a central processing unit (CPU) 403. TheASIC 402 and the CPU 403 are connected to each other via an address databus. The ASIC 402 includes circuits for generating pseudo BD signals.

First, a BD signal 401 serving as a horizontal synchronizing signal fromthe BD sensor 110 is input to the ASIC 402 provided in the enginecontroller 204 and the video controller 203. The ASIC 402 receives theBD signal 401, calculates a BD period, and transmits a value of thecalculated BD period to the CPU 403. The BD period represents a intervalbetween laser beams sequentially detected by the BD sensor 110.

The CPU 403 corrects the BD signal 401 from the value of the BD periodto calculate a correction value for generating a pseudo BD signal, andinputs the correction value to the ASIC 402 via the address data bus.The ASIC 402 generates (outputs) a pseudo BD signal 404 based on thecorrection value and the BD signal 401 from the BD sensor 110. Theoutput pseudo BD signal 404 is input to the video controller 203.

The correction value is used for correcting a difference between atiming to start to form the electrostatic latent image based on the BDsignal 401 for the laser diode LD2 and a timing to start to form theelectrostatic latent image based on the BD signal 401 for the laserdiode LD4. The CPU 403 corresponds to a calculation unit configured tocalculate the correction value.

The video controller 203 receives the BD signal 401 output from the BDsensor 110 and the pseudo BD signal 404 output from the ASIC 402. Imagedata VDOM and VDOY are respectively output to the laser diodes LD3 andLD4 in the scanner unit 205 from the video controller 203 at apredetermined timing after the BD signal 401 is input. The laser diodesLD3 and LD4 respectively emit light based on the image data VDOM andVDOY so that respective electrostatic latent images based on the imagedata VDOM and VDOY are formed on the photosensitive drums 303 and 304.Similarly, image data VDOK and VDOC are respectively output to the laserdiodes LD1 and LD2 in the scanner unit 205 from the video controller 203at a predetermined timing after the pseudo BD signal 404 is input. Thelaser diodes LD1 and LD2 respectively emit light based on the image dataVDOK and VDOC so that electrostatic latent images based on the imagedata VDOK and VDOC are formed on the photosensitive drums 301 and 302.

FIG. 4 is a timing chart for illustrating an operation in the firstexemplary embodiment. In the polygon mirror 105, the BD period differsfor each of the mirror surfaces due to an error in forming accuracy(plane division error). That is, intervals among the respective laserbeams reflected from the mirror surfaces of the polygon mirror 105 andsequentially detected by the BD sensor 110 differ.

In the present exemplary embodiment, the polygon mirror 105 serving asthe rotating polygon mirror is square, as viewed from its axis ofrotation, and has four mirror surfaces on its side surfaces. The fourrespective mirror surfaces are referred to as an A surface, a B surface,a C surface, and a D surface.

In the present exemplary embodiment, the BD signal 401 has a BD periodxa for the A surface to the B surface, a BD period xb for the B surfaceto the C surface, a BD period xc for the C surface to the D surface, anda BD period xd for the D surface to the A surface of the polygon mirror105, which have been measured by the ASIC 402. For example, the BDperiod xa can be a period of time that has passed until the BD sensor110 detects the laser beam emitted from the laser diode LD4 andreflected from the B surface since the BD sensor 110 has detected thelaser beam emitted from the laser diode LD4 and reflected from the Asurface (an interval between the laser beams detected by the BD sensor110). In the present exemplary embodiment, the shortest BD period(corresponding to a reference interval in the present invention) amongthe four BD periods is subtracted from the BD period for each of themirror surfaces, and a value obtained by the subtraction is used as acorrection value.

The reason is as follows. When the A surface is used for the BD signal401, the B surface is used for the pseudo BD signal 404. When the Bsurface is used for the BD signal 401, the C surface is used for thepseudo BD signal 404. When the C surface is used for the BD signal 401,the D surface is used for the pseudo BD signal 404. When the D surfaceis used for the BD signal 401, the A surface is used for the pseudo BDsignal 404. The correction value is determined from a correspondencebetween the BD signal 401 and the pseudo BD signal 404. The correctionvalue depends on the polygon mirror 105 and hardly changes with time.Thus, starting the writing from the BD signal 401 remains the same. Whenit is determined that the correction value is zero for the mirrorsurface corresponding to the shortest BD period, the reference mirrorsurface is determined.

Accordingly, when the shortest BD period is xb, the correction value isas follows. A correction value for the pseudo BD signal 404 for the Bsurface corresponding to the BD signal 401 for the A surface is given bythe following equation:

(Period for A surface to B surface of BD signal)−(Shortest BDperiod)=xa−xb

Therefore, the correction value is xa−xb.

A correction value for the pseudo BD signal 404 for the C surfacecorresponding to the BD signal 401 for the B surface is given by thefollowing equation:

(Period for B surface to C surface of BD signal)−(Shortest BDperiod)=xb−xb

Therefore, the correction value is 0.

A correction value for the pseudo BD signal 404 for the D surfacecorresponding to the BD signal 401 for the C surface is given by thefollowing equation:

(Period for C surface to D surface of BD signal)−(Shortest BDperiod)=xc−xb

Therefore, the correction value is xc−xb.

A correction value for the pseudo BD signal 404 for the A surfacecorresponding to the BD signal 401 for the D surface is given by thefollowing equation:

(Period form D surface to A surface of BD signal)−(Shortest BDperiod)=xd−xb

Therefore, the correction value is xd−xb.

The correction value for the pseudo BD signal 404 (the pseudo BD signal404 for the B surface) corresponding to the BD signal 401 for the Asurface is xa−xb. Thus, the pseudo BD signal 404, which is delayed by(xa−xb) from the BD signal 401, is generated and output.

The correction value for the pseudo signal 404 (the pseudo BD signal 404for the C surface) corresponding to the BD signal 401 for the B surfaceis 0. Thus, the BD signal 401 itself is output as the pseudo BD signal404.

The correction value for the pseudo BD signal 404 (the pseudo BD signal404 for the D surface) corresponding to the BD signal 401 for the Csurface is xc−xb. Thus, the pseudo BD signal 404, which is delayed by(xc−xb) from the BD signal 401, is generated and output.

The correction value for the pseudo BD signal 404 (the pseudo BD signal404 for the A surface) corresponding to the BD signal 401 for the Dsurface is xd−xb. Thus, the pseudo BD signal 404, which is delayed by(xd−xb) from the BD signal 401, is generated and output.

Thus, the pseudo BD signal 404 is a signal whose timing is delayed bythe correction value from the BD signal 401. The pseudo BD signal 404 isoutput at a timing delayed by a period of time based on the correctionvalue from a timing at which the BD signal 401 is output.

In the generation of the pseudo BD signal, the mirror surface of thepolygon mirror 105 which the laser beam emitted from the laser diode LD4is emitted enters is determined when the BD signal 401 is output. Thus,the correction value is determined from the BD period calculated tocorrespond to the determined mirror surface. At this time, the BD perioddiffers for each of the mirror surfaces of the polygon mirror 105. Thus,it is possible to specify on which of the mirror surfaces the laser beamfrom the laser diode LD4 is deflected by calculating the BD period. Inthe case of the BD signal 401, the pseudo BD signal 404 as illustratedin FIG. 4 is generated. The foregoing is a series of processes forgenerating the pseudo BD signal.

<Pseudo BD Signal Generation Circuit>

A configuration of circuits provided in the ASIC 402 to generate thepseudo BD signal 404 will be described below with reference to FIGS. 5and 6. FIG. 5 is a block diagram illustrating a circuit configuration ofthe ASIC 402 for generating the pseudo BD signal 404 in the firstexemplary embodiment. FIG. 6 is a timing chart for determiningrespective positions by the circuits in the ASIC 402.

First, the BD signal 401 output from the BD sensor 110 and a poristart702 serving as a signal for starting control to generate the pseudo BDsignal 404 (hereinafter referred to as pseudo BD control) are input to a2-bit counter 701. The poristart 702 is input using the CPU 403 and anADDRESSDATABUS 23 serving as a signal line to the ASIC 402 to start thepseudo BD control. The 2-bit counter 701 repeats its count values (DATA)00, 01, 11, and 10 in this order so as to recognize which of the mirrorsurfaces of the polygon 105 is irradiated with a laser.

If the A surface is irradiated with the laser when the count value(DATA) is 00, the B surface, the C surface, and the D surfacerespectively are irradiated with the laser when 01, 11, and 10. When theBD period for the A surface is measured, a sela 703 enters a High level,as illustrated in the timing chart for determining respective positionsof polygonal surfaces by the circuits in the ASIC 402 illustrated inFIG. 6. When the BD period for the B surface is measured, a selb 704enters a High level. When the BD period for the C surface is measured, aselc 705 enters a High level. When the BD period for the D surface ismeasured, a seld 706 enters a High level.

A 17-bit counter 707 counts the BD period using a clk 722. When the sela703, the selb 704, the selc 705, and the seld 706 are selected, itscount values DATA00, DATA01, DATA11, and DATA10 of the BD periods forthe mirror surfaces of the polygon 105 respectively are added 32 timesto signs 708, 709, 710, and 711.

To divide, by 32, the count values DATA00, DATA01, DATA11, and DATA10 ofthe BD periods, which have been added 32 times, to calculate averagevalues per period, the added count values DATA00, DATA01, DATA11, andDATA10 are shifted downward by 5 bits (712), and their respectivehigh-order 5 bits are deleted. Respective obtained count values arestored in 17-bit registers 713, 714, 715, and 716. When it is detectedthat a 5-bit counter 717 has added the count values of the BD periodsfor the mirror surfaces of the polygon 105 32 times, a poriend 718serving as a BD period addition end signal is output. Contents of the17-bit registers 713, 714, 715, and 716 are the average values of the BDperiods. When the poriend 718 is output, the CPU 403 can read averagevalues xa, xb, xc, and xd of the BD periods from 32 times of additionusing the ADDRESSDATABUS 723. The CPU 403 can also read the poriend 718using the ADDRESSDATABUS 723. If it is detected that the poriend 718 isoutput, therefore, the CPU 403 reads the average values xa, xb, xc, andxd of the BD periods.

The CPU 403 then inputs correction values xas, xbs, xcs, and xdscorresponding to the respective mirror surfaces to 8-bit registers 718,719, 720, and 721 in the ASIC 402 via the ADDRESSDATABUS 723. Each ofthe sela 703, selb 704, selc 705, and seld 706 selects any correctionvalue. A 8-bit counter 722 outputs the pseudo BD signal 404 to the videocontroller 203 from the correction values xas′, xbs′, xcs′, and xds′. Inthe present exemplary embodiment, the correction value is calculatedfrom the average value of the BD period for each of the mirror surfacesof the polygon 105 from 32 times of addition. However, the number oftimes of addition is not limited to this. If the BD period for each ofthe mirror surfaces is added 64 times, a count value of the BD periodmay be shifted downward by 6 bits, and its high-order 5 bits may bedeleted.

The foregoing is a description of an internal circuit configuration ofthe ASIC 402. While the generation of the pseudo BD signal 404 has beendescribed with reference to FIG. 5, respective circuits for generatingpseudo BD signals 405 and 406 are also provided in the ASIC 402, and thepseudo BD signals 405 and 406 are generated in a similar method to themethod for generating the pseudo BD signal 404.

<Print Sequence>

A print sequence according to the present exemplary embodiment will bedescribed below. In the present exemplary embodiment, control togenerate a pseudo BD signal (pseudo BD control) during the startup ofthe scanner unit 205 is performed, to shorten a first print output timein a full-color mode. A configuration in which the laser diode LD3(magenta M), and the other laser diodes LD4 (yellow Y), LD2 (cyan C),and LD1 (black BK) align images in the main scanning direction bycontrolling light emission timings based on a BD signal and a pseudo BDsignal, respectively will be described by way of example. A period oftime during which the scanner unit 205 is being started up means aperiod of time that has passed until the polygon mirror 105 reachessteady rotation since it has started to be driven to rotate.

FIG. 7 is a timing chart for illustrating print sequences in the presentexemplary embodiment and a comparative example, where FIGS. 7A and 7Brespectively are timing charts illustrating a case where pseudo BDcontrol is performed after a scanner unit in the comparative example isstarted up and a case where pseudo BD control is performed while thescanner unit in the present exemplary embodiment is being started up.The horizontal axis and the vertical axis respectively indicate anelapse of time from the start of printing, and processing sequentiallyperformed by the image forming apparatus. The number of elements andtimes required for the elements in the figures in the present exemplaryembodiment are similar to those in the conventional example. FIGS. 7Aand 7B illustrate a case where color printing is performed on onerecording medium.

The timing chart in the comparative example illustrated in FIG. 7A willbe first described. At T101, the image forming apparatus first starts tostart up a scanner motor, start up a fixing device, and start up ahigh-voltage power source upon receiving a print instruction. Thestartup of the high-voltage power source is to perform control such thata voltage and a current of the high-voltage power source for each ofcharging, development, and transfer required for an electrophotographicprocess reach target values. When the startup of the scanner motor andthe startup of the high-voltage power source end, the image formingapparatus starts pseudo BD control at T102. After the pseudo BD controlends at T103, the image forming apparatus starts formation of an imagein four colors (yellow Y, magenta M, cyan C, and black Bk) and primarytransfer. When this processing ends, the image forming apparatusperforms secondary transfer for transferring a toner image formed on theintermediate transfer belt 211 onto a recording medium at T104. At T105,the image forming apparatus uses the fixing device, which has beencontrolled to a target temperature, then fixes the toner image, whichhas been transferred onto the recording medium, as a permanent image onthe recording medium. When the fixing ends, the image forming apparatusdischarges the recording medium onto a sheet discharge tray at T106, andthe image formation ends at T107.

The timing chart in the first exemplary embodiment illustrated in FIG.7B will be described below. A difference between the comparative exampleillustrated in FIG. 7A and the first exemplary embodiment illustrated inFIG. 7B is a timing to start pseudo BD control. The timing to start thepseudo BD control is after the startup of the scanner motor in FIG. 7Awhile it is a timing (T200) at which the startup of the scanner motor isstarted upon receiving a print instruction in FIG. 7B. Thus, in FIG. 7B,the image forming apparatus is performing the pseudo BD control duringthe startup of a scanner motor. Details of the pseudo BD control duringthe startup of the scanner motor will be described below. Control of thesubsequent secondary transfer, fixing, and discharge is similar to thatillustrated in FIG. 7A, and hence description thereof is not repeated.As a result, a timing to start formation of an image in yellow Y is thetime when the pseudo BD control ends (T103) in FIG. 7A while being atiming (T202) at which the startup of the scanner motor is completed inFIG. 7B.

A first print output time is a period of time that has passed until theimage forming apparatus forms an image on the first recording medium anddischarges the recording medium to the outside of the image formingapparatus since it has received a print instruction. The first printoutput time is a period of time from T100 to T107 when formation of animage in yellow is started after completion of generation of a pseudo BDsignal. The first print output time is a period of time from T200 toT207 when formation of an image in yellow is started before completionof generation of the pseudo BD signal. As can be seen from FIGS. 7A and7B, the first print output time in the present exemplary embodiment ismade shorter by a time (Ts) required for the pseudo BD control than inthe comparative example.

<Pseudo BD Control During Startup of Scanner>

A specific method for implementing pseudo BD control during the startupof the scanner unit 205 will be described with reference to FIGS. 8 and9. FIG. 8 illustrates the acceleration of the polygon mirror 105. FIG. 9is a graph illustrating a relationship between a speed and a time whenthe polygon mirror 105 is driven to rotate.

The time when the startup of the scanner unit 205 ends is the time whenthe polygon mirror 105 enters a steady rotation state (a state where arotation speed V converges in a predetermined speed range in which imageformation can be performed. In the steady rotation state, a period oftime required for a photosensitive drum to be scanned from end to endwith a laser beam is substantially constant. In this case, a correctionvalue corresponding to each of the mirror surfaces of the polygon mirror105 can be calculated by measuring BD periods for the mirror surfaces ofthe polygon mirror 105 and using the shortest BD period as a reference,as already be described.

However, the number of revolutions of the motor of the scanner unit 205increases with a lapse of time during the startup of the scanner unit205. Thus, a period of time during which the mirror surfaces of thepolygon mirror 105 is scanned with a laser beam decreases. Therefore,each of the mirror surfaces cannot be scanned with the laser beam at apredetermined speed. Thus, the correction value cannot be calculated inthe above-mentioned method for calculating the correction value. Amethod for calculating the correction value during the startup of thescanner unit 205 will be described below.

When control of the startup of the scanner unit 205 is started, thepolygon mirror 105 starts to be driven to rotate, as indicated by asection Tc illustrated in FIG. 9, and the number of revolutions of thepolygon mirror 105 increases with a lapse of time.

As illustrated in FIG. 8B, a distance between a point O serving as thecenter of the polygon mirror 105 and a point X serving as a positionwhere the laser beam enters the BD sensor 110 when the A surface of thepolygon mirror 105 is scanned with the laser beam is defined as r. Asillustrated in FIG. 8C, a locus drawn by the point X when the polygonmirror 105 rotates is a circle E. The circumference of the circle E is2πr when the radius of the circle E is r.

A speed at which the point X moves on the circle E is defined as v, aspeed v at a certain time t is at (a is a constant), the time when theBD signal for the A surface is detected is defined as ta, and the timewhen the BD signal for the B surface is detected is defined as tb. Inthis case, an area S1 illustrated in FIG. 9 represents a distance from aposition where the BD signal for the A surface enters the BD sensor 110to a position where the BD signal for the B surface enters the BD sensor110. S1 is expressed by the following equation 1:

S1=∫_(t) _(a) ^(t) ^(b) atdt  (Equation 1)

According to (Equation 1), even if the number of revolutions of thepolygon mirror 105 during the startup of the scanner unit 205 increases,a distance (an interval) between the BD signals during steady rotationof the polygon mirror 105 can be obtained by calculation. Similarly, aperiod for the A surface to the B surface, a period for the B surface tothe C surface, a period for the C surface to the D surface, and a periodfor the D surface to the A surface of the BD signal can be respectivelyobtained. Thus, the correction value corresponding to each of the mirrorsurfaces of the polygon mirror 105 can be calculated by theabove-mentioned method for calculating the correction value.

The constant a represents an acceleration of the point X moving on thecircumference of the circle E, and is obtained by a method describedbelow. During the startup of the scanner unit 205, the point X moveswhile increasing the speed v with the time t on the circumference (2πr)of the circle E. A period of time required for the point X to move onthe circumference (2πr) is obtained from the number of times the BDsensor 110 has detected the BD signal and the time when the BD sensor110 has detected the BD signal. Accordingly, assuming that the BD signalat any timing in the section Tc where the scanner unit 205 is beingstarted up after starting the startup is the BD signal for the Asurface, a period of time required for the point X to move on thecircumference (2πr) in the first revolution is defined as t1, and anaverage speed of the point X that is moving on the circumference isdefined as v1. Similarly, a period of time required for the point X tomove on the circumference in the second revolution and an average speedof the point X respectively are defined as t2 and v2, . . . , and aperiod of time required for the point X to move on the circumference inthe n-th revolution and an average speed of the point X respectively aredefined as to and vn.

2 Π r = t 1 × v 1  in  first  revolution2 Π r = t 2 × v 2  in  second  revolution …2 Π r = tn × vn  in  n-th  revolution

The acceleration a is represented by a rate of change of a speed perunit time. Thus, accelerations a1 to an−1 can be expressed as follows:

a 1 = (v 2 − v 1)/t 2 a 2 = (v 3 − v 2)/t 3 …an − 1 = ((vn) − (vn − 1))/tn

As described above, the acceleration a may be obtained by averaging theabove-mentioned accelerations a1 to an−1. The acceleration a can beobtained by the foregoing calculation. Thus, a BD period for each of themirror surfaces of the polygon mirror 105 that is being accelerated (adetection interval between laser beams) can be obtained by calculationusing (Equation 1).

As described above, according to the first exemplary embodiment, the BDperiod for each of the mirror surfaces of the polygon mirror 105 canalso be calculated during the startup of the scanner unit 205, and thepseudo BD signal suitable for each of the mirror surfaces of the polygonmirror 105 can be generated by calculating the correction value.

Furthermore, a flow of the pseudo BD control during the startup of thescanner unit 205 will be described with reference to FIG. 10. FIG. 10 isa flowchart for illustrating the pseudo BD control.

In step S100, the pseudo BD control is started when control of thestartup of the scanner unit 205 is started. In step S101, the number ofrevolutions n of the polygon mirror 105 is set to zero. In step S102, itis determined whether the polygon mirror 105 has made one revolutionfrom the number of times the BD signal has been detected. If the polygonmirror 105 has made one revolution (YES in step S102), then in stepS103, the number of revolutions n of the polygon mirror 105 is set to(n+1). In step S104, it is determined whether the number of revolutionsn of the polygon mirror 105 is more than 1. If the number of revolutionsn of the polygon mirror 105 is more than 1 (YES in step S104), then instep S105, an acceleration an−1 is calculated. In step S106, theacceleration an−1 is added to a buffer provided in a random accessmemory (RAM) in the CPU 403.

In step S107, it is then determined whether the number of revolutions nof the polygon mirror 105 has reached a predetermined number ofrevolutions. “The number of revolutions of the polygon mirror 105 hasreached the predetermined number of times” is referred to as “thepolygon mirror 105 has been scanner ready”. A speed at the time when thepolygon mirror 105 has been “scanner ready” is defined as Vrdy. If thepolygon mirror 105 has been “scanner ready” (YES in step S107), then instep S108, a=buffer/(n−1) is calculated, to obtain the acceleration a.

In step S109, a distance (interval) between the BD signals for themirror surfaces of the polygon mirror 105 is then obtained from(Equation 1), to set the mirror surface for which the distance betweenthe BD signals is the shortest to a reference surface. A BD periodbetween the BD signals is calculated from the distance between the BDsignals for the mirror surfaces and the speed Vrdy. In step S110, acorrection value is calculated by subtracting the BD period for thereference surface from the BD period for each of the mirror surfaces, asdescribed above. In step S111, the pseudo BD control then ends.

As described above, according to the first exemplary embodiment, whenthe scanner unit 205 is started up (while the polygon mirror 105 isbeing accelerated), the correction value can also be calculated bycalculating the BD period for each of the mirror surfaces of the polygonmirror 105 based on the output of the BD sensor 110. Accordingly, theimage formation can be started immediately after the polygon mirror 105enters a steady rotation state. Therefore, the first print output timecan be made short compared with a configuration in which the BD periodis measured to generate the pseudo BD signal after the polygon mirror105 reaches steady rotation. The correction value may be calculated atleast based on the BD signal output while the polygon mirror 105 isbeing accelerated or may be calculated based on the BD signal outputwhile the polygon mirror 105 is being accelerated and the BD signaloutput after the polygon mirror 105 reaches steady rotation. In thiscase, the correction value for the pseudo BD signal can be calculatedmore early and the first print output time can be made short comparedwith in a configuration in which measurement of the BD period is startedto generate the pseudo BD signal after the polygon mirror 105 reachessteady rotation.

A second exemplary embodiment of the present invention will be describedbelow with reference to FIGS. 11 and 12. FIG. 11 illustrates aconfiguration of a system for generating a pseudo BD signal in thesecond exemplary embodiment. FIG. 12 is a block diagram illustrating aconfiguration of an ASIC in the second exemplary embodiment. The videocontroller 203 includes the ASIC 404 and the CPU 405. The ASIC 404 andthe CPU 405 are connected to each other via an address data bus. TheASIC 404 includes circuits for generating pseudo BD signals serving as asecond signal.

The BD signal 401 serving as a first signal serving as a horizontalsynchronizing signal from the BD sensor 110 serving as a detection unitis input to the ASIC 404 provided in the video controller 203. The videocontroller 203 starts pseudo BD control when it is notified to startcontrol of the startup of the scanner unit 205 from an engine controller204 via serial communication. The ASIC 404 receives the BD signal 401,and calculates a BD period. The CPU 405 calculates a correction valuefor a pseudo BD signal from the BD period. The correction value is inputto the ASIC 404 via the ADDRESSDATABUS 723.

The ASIC 404 can generate pseudo BD signals (not illustrated) for lightsources other than a light source provided with the BD sensor 110. Whenthe startup of the scanner unit 205 is completed, the engine controller204 notifies the video controller 203 that the startup of the scannerunit 205 is completed, via serial communication. The video controller203 notifies the engine controller 204 of the end of the pseudo BDcontrol at a timing at which the pseudo BD control has ended.

The engine controller 204 outputs a /TOP signal serving as a referencetiming to output a video signal, and the video controller 203 outputsimage data VDOK, VDOC, VDOM, and VDOY to the laser diodes LD1, LD2, LD3,and LD4 in the scanner unit 205, respectively. Respective images areformed on the intermediate transfer belt 211, and is printed on arecording medium based on the image data VDOK, VDOC, VDOM, and VDOY.

An internal circuit configuration of the ASIC 404 will be describedbelow with reference to FIG. 12. The BD signal 401 and the poristart 702serving as a signal using the CPU 405 and the ADDRESSDATABUS 723 servingas a signal line to the ASIC 404 to start the pseudo BD control areinput to the 2-bit counter 701. The poristart 702 is a signal forstarting the pseudo BD control. The 2-bit counter 701 repeats its countvalues (DATA) 00, 01, 11, and in this order so as to recognize which ofmirror surfaces of a polygon mirror 105 is irradiated with a laser.

If the A surface is irradiated with the laser when the count value(DATA) is 00, the B surface, the C surface, and the D surface,respectively are irradiated with the laser when 01, 11, and 10.Consequently, when a BD period for the A surface is measured, the sela703 enters a High level, as illustrated in the timing chart fordetermining respective positions of polygonal surfaces by the circuitsin the ASIC 402 illustrated in FIG. 6. When a BD period for the Bsurface is measured, the selb 704 enters a High level. When a BD periodfor the C surface is measured, the selc 705 enters a High level. When aBD period for the D surface is measured, the seld 706 enters a Highlevel.

A 17-bit counter 707 then counts the BD period using the clk 722. Whenthe sela 703, the selb 704, the selc 705, and the seld 706 are selected,its count values DATA00, DATA01, DATA11, and DATA10 of the BD periodsfor the mirror surfaces of the polygon 105 are added 32 times to thesigns 708, 709, 710, and 711, respectively. To divide, by 32 the countvalues DATA00, DATA01, DATA11, and DATA10 of the BD periods, which havebeen added 32 times, to calculate average values per period, the addedcount values DATA00, DATA01, DATA11, and DATA10 are shifted downward by5 bits (712), and their respective high-order 5 bits are deleted.Respective obtained count values are stored in the 17-bit registers 713,714, 715, and 716. When it is detected that the 5-bit counter 717 hasadded the count values of the BD periods for the mirror surfaces of thepolygon 105 32 times, the poriend 718 serving as a BD period additionend signal is output.

Contents of this 17-bit registers 713, 714, 715, and 716 are the averagevalues of the BD periods. When the poriend 718 is output, the CPU 405can read average values xa, xb, xc, and xd of the BD periods from 32times of addition using the ADDRESSDATABUS 723. The CPU 405 can alsoread the poriend 718 using the ADDRESSDATABUS 723. If it is detectedthat the poriend 718 is output, therefore, the CPU 405 reads the averagevalues xa, xb, xc, and xd of the BD periods.

The CPU 405 then inputs correction values xas, xbs, xcs, and xdscorresponding to the respective polygonal surfaces to 8-bit registers718, 719, 720, and 721 in the ASIC 404 via the ADDRESSDATABUS 723. Eachof the sela 703, selb 704, selc 705, and seld 706 selects any correctionvalues. A 8-bit counter 722 outputs the pseudo BD signal 404 from thecorrection values xas′, xbs′, xcs′, and xds′. In the present exemplaryembodiment, the correction value is calculated from the average valuesof the BD periods for each of the mirror surfaces of the polygon 105from 32 times of addition. However, the number of times of addition isnot limited to this. If the BD period for each of the mirror surfaces isadded for 64 times, a count value of the BD period may be shifteddownward by 6 bits, and its high-order 5 bits may be deleted.

The foregoing is a description of a circuit block diagram in the ASIC404 in the second exemplary embodiment. While the generation of thepseudo BD signal 404 has been described with reference to the circuitblock diagram, similar respective circuit blocks for generating thepseudo BD signal 405 and the pseudo BD signal 406 are also provided inthe ASIC 404, and the pseudo BD signals 405 and 406 are generated in amethod similar to the method for generating the pseudo BD signal 404.

A method for calculating the BD period for each of the mirror surfacesof the polygon mirror 105 during the startup of the scanner unit 205 anda method for calculating the correction value corresponding to each ofthe mirror surfaces of the polygon mirror 105 are similar to the methodsin the first exemplary embodiment. Details of the pseudo BD controlduring the startup of the scanner unit 205 are illustrated in theflowchart illustrated in FIG. 10, like in the first exemplaryembodiment. In step S100, the video controller 203 first starts thepseudo BD control when it is notified to start control of the startup ofthe scanner unit 205 by the engine controller 204. The subsequent flowis similar to that in the first exemplary embodiment.

As described above, according to the second exemplary embodiment, the BDperiod for each of the mirror surfaces of the polygon mirror 105 canalso be calculated during the startup of the scanner unit 205, and thevideo controller 203 generates the pseudo BD signal. Thus, the imageformation can be started immediately after the polygon mirror 105reaches steady rotation. Therefore, the first print output time can beshortened.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-031723 filed Feb. 21, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus comprising: a firstlight source configured to emit a first laser beam and a second lightsource configured to emit a second laser beam; a first image bearingmember and a second image bearing member configured to bear a developerimage; a rotating polygon mirror configured to rotate and reflect thefirst laser beam and the second laser beam to scan the first bearingmember with the first laser beam and scan the second bearing member withthe second laser beam, a mirror surface reflecting the second laser beamis different from a mirror surface reflecting the first laser beam atthe same time, a direction of reflecting the second laser beam isdifferent from a direction of reflecting the first laser beam at thesame time; a detection unit configured to detect the first laser beamreflected from each of the mirror surfaces by rotation of the rotatingpolygon mirror, to output a signal; a first generation unit configuredto generate a first signal for determining a timing to form anelectrostatic latent image on the first image bearing member with thefirst laser beam based on timings at which the respective first laserbeams, which have been reflected from the mirror surfaces, aresequentially detected by the detection unit; a second generation unitconfigured to generate a second signal for determining a timing to forman electrostatic latent image on the second image bearing member withthe second laser beam reflected from the mirror surface different fromthe mirror surface reflecting the first laser beam based on the firstsignal and a correction value; and a calculation unit configured tocalculate the correction value based on the output of the detectionunit, wherein the calculation unit calculates the correction value basedon the signal output from the detection unit in a period of time thathas passed until the rotating polygon mirror reaches steady rotationsince it has started to be driven to rotate.
 2. The image formingapparatus according to claim 1, wherein the calculation unit obtainseach of intervals among the respective laser beams reflected from themirror surfaces during the steady rotation of the rotating polygonmirror and sequentially detected by the detection unit from anacceleration of the rotation of the rotating polygon mirror, andcalculates the correction value from a difference between each of theintervals and a reference interval among the intervals.
 3. The imageforming apparatus according to claim 2, wherein the reference intervalis a shortest interval among the intervals.
 4. The image formingapparatus according to claim 1, wherein the second signal is output at atiming delayed by a period of time based on the correction value from atiming at which the first signal is output.
 5. The image formingapparatus according to claim 1, wherein the detection unit is a sensorarranged on an optical path of the first laser beam reflected from themirror surface of the rotating polygon mirror.
 6. The image formingapparatus according to claim 1, wherein the rotating polygon mirror hasthe four mirror surface.
 7. The image forming apparatus according toclaim 1, further comprising: a charging unit configured to chargerespective surfaces of the first image bearing member and the secondimage bearing member; and a development unit configured to supply adeveloper to the electrostatic latent image formed by scanning the firstimage bearing member and the second image bearing member charged by thecharging unit with the first laser beam and the second laser beam, toform the developer image.
 8. The image forming apparatus according toclaim 1, further comprising an endless belt configured to contact eachof the first image bearing member and the second image bearing member.